Polyurethane foams are used in a wide variety of applications, ranging from cushioning (such as mattresses, pillows and seat cushions) to packaging, shoe soling, to thermal insulation and for medical and automotive applications. One class of polyurethane foam is known as viscoelastic (VE) or “memory” foam. Viscoelastic foams exhibit a time-delayed and rate-dependent response to an applied stress. They have low resiliency, open cells and recover slowly when compressed. These properties are often associated with the glass transition temperature (Tg) of the polyurethane. Viscoelasticity is often manifested when the polymer has a Tg at or near the use temperature, which is room temperature for many applications, such as acoustic in automotive application or comfort in furniture application.
Viscoelastic foam can be produced in slabstock or in molding, either as a pad or as part of a composite, for instance with a heavy layer. One type of viscoelastic foams include viscoelastic molded foams with foamed interiors and more compact outer skin layers used directly with a thin fabric cover in furniture applications, such as mattresses, pillows, and medical devices.
Molded viscoelastic polyurethane foams can be formed in an open mold process or a closed mold process. In the open mold process two reactive components are mixed and poured into an open mold and well dispersed onto the mold surface. The mold is then closed and the mixture is allowed to expand and cure. With the closed mold process, the mixed components are injected into a closed mold through an injection point, hence foaming mass has to flow well within the mold. In both cases a release agent may be to be applied by spraying or brushing onto the mold surface including the lid before foam injection (between 10 seconds and 1 minute depending on the process conditions) to prevent the foam to stick to the mold and in order to get a foam skin without surface and/or sub-surface defects, such as pin holes, voids, local collapses, bubbles, blisters, and skin peeling, which may be detrimental for the application, both for aesthetic and for comfort reasons.
Two types of release agents may be used: a solvent based system whereby a solvent is evaporated when in contact with the heat of the mold, hence releasing a waxy layer onto the foam surface, or a water based release agent, where most of water has no time for evaporation before foam injection, hence the release agent has to be formulated properly to avoid this residual water to react with the isocyanate, hence to create local, undesirable gas formation. For environmental reasons, water based release agents are nowadays preferred to reduce volatile organic compounds. In addition to the effect of release agents, the skin formation and quality of viscoelastic foams is governed by foam formulation, controlling nucleation, cell formation, cell stabilization, and gelation. Preferably the skin is smooth and flexible, while at the same time is strong to avoid tearing at demold and during handling and storing.
Like most polyurethane foams, in VE polyurethane foams the two reactive components are a polyol component and a polyisocyanate component. The two components may be reacted in the presence of a blowing agent. The blowing agent is usually water or, less preferably, a mixture of water and another material. The predominant polyol used in these formulations has a functionality of about 3 hydroxyl groups/molecule and a molecular weight in the range of 400-1500. This polyol composition is primarily the principal determinant of the Tg (Glass Transition Temperature) of the polyurethane foam, although other factors such as water levels and isocyanate type and index also play significant roles. However, as mentioned above, the skin of molded viscoelastic pads may have surface defects and/or sticky surfaces upon being demolded, with both solvent and water based release agents, resulting in a high scrap rate.
Therefore, there is a need for slab stock foam without internal collapses, to maximize production yield.